1. Field of the Invention
This invention relates to a primer for use in the fluidized-bed dip-coating of thermoplastic resin powder.
2. Description of Prior Art
Powder coating methods are considered excellent from the viewpoints of resource savings, prevention of environmental pollution and excellent coating properties.
These powder coating methods are divided into two types according to the application field, namely, decorative coating and functional coating for imparting to the coated object such properties as corrosion resistance, electric insulation, abrasion resistance and weatherability. For functional coating, the process of fluidized-bed dip-coating of thermoplastic resin powder is widely employed. In this process, the coating is applied in a thickness of several hundred microns and the using conditions employed are generally very severe. Accordingly, in addition to degradation of the coating, the frequent occurrence of peeling is a great problem in the art of fluidized-bed dip-coating of thermoplastic resin powder.
This invention relates to a primer for preventing peeling of a thermoplastic resin coating, formed by the fluidized-bed dip-coating of thermoplastic resin powder, from a metal substrate. More particularly, this invention relates to a primer having a good resistance to hot water which otherwise promotes extremely the peeling of such a coating from a metal substrate.
A compound formed by polymerizing a diene compound has already been used as a primer for a thermoplastic resin powder, especially nylon powder. However, as is seen from the results of Comparative Example 1 given in Table 1 hereinafter, the boiling water resistance of a polymeric primer formed by polymerizing a diene compound is about 20 to about 30 hours, and this value cannot be said to be a sufficient value.
We performed research with a view to improving the boiling water resistance of polymeric primers formed by polymerizing diene compounds, but all attempts based on conventional chemical knowledge resulted in failure. For example, substances considered to have some influence on the chemical reactions of polymeric compounds formed by polymerizing diene compounds, such as radical initiators, oxidation promotors, antioxidants and peroxide decomposing agents, were found to have no effect of improving the boiling water resistance. Further, substances having an influence on the interfacial phenomenon, such as silicone oils and silane coupling agents, were found to have no effect.
Still further, fillers generally incorporated in primers or coatings, such as aluminum powder, titanium oxide, talc, hollow glass beads, zinc powder, graphite powder, aluminum oxide powder, antimony oxide and diatomaceous earth, were found to have no effect at all.